Move middle-click autoscroll to synthetic fling.

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index 6ff1983d7139c16be4b7604726a8c80db40dea7f..ce1439dd1eef3826cb7cb9c412fe76faf8b1e306 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -1340,7 +1340,9 @@ void InputHandlerProxy::Animate(base::TimeTicks time) {
     TRACE_EVENT_INSTANT0("input",
                          "InputHandlerProxy::animate::flingOver",
                          TRACE_EVENT_SCOPE_THREAD);
-    CancelCurrentFling();
+    if (fling_parameters_.source_device !=
+        blink::kWebGestureDeviceSyntheticAutoscroll)
+      CancelCurrentFling();
   }
 }